The present invention relates to an apparatus for treating an exhaust gas from a nuclear plant, and more particularly to an apparatus for recovering radioactive gases especially Kr-85 from an exhaust gas of a nuclear plant.
With an increase in necessity of treating an exhaust gas discharged from nuclear plants such as nuclear power plants, used nuclear reprocessing plants, etc., in order to recover dangerous radioactive gases such as Kr-85, there have been tried developments of apparatus for treating the exhaust gas.
In a used nuclear fuel, various radioactive elements as fission products due to nuclear fission are included. It is undesirable to abandon them because it raises the operating cost of a nuclear plant. Accordingly, it is usual to reprocess the used nuclear fuel and thereby to obtain useful nuclear elements.
In reprocessing a used nuclear fuel, a fuel material is dissolved in an acid such as nitric acid and the resulting acid solution of fuel material is chemically purified to eliminate compounds composed of unnecessary elements therefrom.
In such a process, an exhaust gas which includes radioactive gases is generated. Particularly, since the gas of Kr-85 is a radioactive gas having a half life as long as about 10 years, the exhaust gas cannot be discharged directly into the atmosphere. For this reason, it is necessary to recover at least Kr-85 gas in radioactive gases generated in reprocessing. The present inventors have found that Kr-85 gas is separated efficiently from the exhaust gas by a cryogenic distillation method. To that effect, a cryogenic distillation device comprising a liquefying device, a continuous distillation tower and a batch distillation tower are enclosed in a casing (cold box) which is packed with an adsorbent.
Recovery of radioactive substances (hereinafter represented as Kr-85 gas) from an exhaust gas is effected as follows. Firstly, Kr-85 gas is separated from the exhaust gas by the continuous distillation tower, and concentration of Kr-85 is highly increased by the batch distillation tower. The concentrated Kr-85 gas is compressed and stored in a reservoir such as a pressure vessel. The cryogenic distillation apparatus serving for an air liquefying separation is provided with a safety valve, etc., but sometimes an explosion occurs on account of hydrocarbon and nitrogen oxides, etc., which are contained in the exhaust gas. In such an event, the casing of the apparatus must not be destroyed, but liquefied Kr-85 in the distillation tower evaporates and the inside of the casing is filled with the evaporated Kr-85 gas. Also a gas-leak accident may occur due to destruction of welded portions in the casing. Since the liquefying and distillation apparatus is provided with tubes for supplying liquid nitrogen to the liquefying apparatus, distillation apparatus, etc., it is not easy to completely seal it hermetically. Therefore, the evaporated Kr-85 gas leaks to the outside of the casing.
In an earlier effort to eliminate the above problem, in which the casing was filled with perlite which is widely used as a heat insulating material, it was found the insulating material could not adsorb the leaked Kr-85 gas. This is because perlite has a small specific surface area of less than 10 m.sup.2 /g and it cannot adsorb Kr-85 effectively.
As described above, where the cryogenic apparatus for liquefying and distillation of the exhaust gas is heat insulated by perlite, the leaked Kr-85 on the occasion of an accident is not prevented from leaking into the atmosphere and this heat insulating material is therefore not useful as a safeguard, insofar as the Kr-85 recovering apparatus is concerned.